Magnetoresistive stacks with an unpinned, fixed synthetic anti-ferromagnetic structure and methods of manufacturing thereof

ABSTRACT

A magnetoresistive magnetic tunnel junction (MTJ) stack includes a free magnetic region, a fixed magnetic region, and a dielectric layer positioned between the free magnetic region and the fixed magnetic region. In one aspect, the fixed magnetic region consists essentially of an unpinned, fixed synthetic anti-ferromagnetic (SAF) structure which comprises (i) a first layer of one or more ferromagnetic materials, including cobalt, (ii) a multi-layer region including a plurality of layers of ferromagnetic materials, wherein the plurality of layers of ferromagnetic materials include a layer of one or more ferromagnetic materials including cobalt, and (iii) an anti-ferromagnetic coupling layer disposed between the first layer and the multi-layer region. The free magnetic region may include a circular shape, the one or more ferromagnetic materials of the first layer may include cobalt, iron and boron, and the dielectric layer may be disposed on the first layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/727,910, filed Jun. 2, 2015 (still pending), which is a divisional ofU.S. patent application Ser. No. 14/303,200, filed Jun. 12, 2014 (nowU.S. Pat. No. 9,093,637), which is a divisional of U.S. patentapplication Ser. No. 13/925,590, filed Jun. 24, 2013 (now U.S. Pat. No.8,754,460), which is a divisional of U.S. patent application Ser. No.11/444,089, filed May 31, 2006 (now U.S. Pat. No. 8,497,538).

FIELD OF THE INVENTION

The present invention generally relates to MRAM devices and moreparticularly to magnetoresistive random access memories having asynthetic anti-ferromagnet structure without a pinning layer.

BACKGROUND OF THE INVENTION

Memories comprise one of the largest markets for semiconductorintegrated circuits. In general, a memory is a storage device thatretains information or data that can be output when needed. Memorydevices are often characterized under such names as high speed, highdensity, or non-volatile memories. A high speed memory, as its nameimplies, is a device having extremely fast read/write times that areuseful in situations where data transfer rates are critical. A highdensity memory has a substantial memory size for large storagecapability. The most common high density solid state memory is a dynamicrandom access memory (DRAM). A non-volatile memory is a memory thatretains information even when power is removed and is thereby apermanent storage medium. A common non-volatile memory is FLASH memory.In general, an ideal memory has characteristics of all of the abovementioned types of memory.

FLASH memory uses charge storage in a floating gate to retaininformation. FLASH memories operate at relatively high voltages, runningcounter to the trend of reducing power supply voltages for other highdensity integrated circuits. Moreover, they have slow program and erasetimes. The ability to write or store charge in the floating gate islimited to a finite number of times that can be exceeded depending onthe application. Memory failure occurs if the maximum number of writesis exceeded. FLASH memory is presently limited for high densityapplications because it cannot be continually scaled to smallerdimensions due to gate oxide limitations.

Another type of non-volatile memory is a magnetoresistive random accessmemory (MRAM). MRAM has the characteristics of an ideal memory as it isa high density memory, is scalable, requires low voltage, and has lowpower consumption and high speed read/write times. A magnetoresistivememory cell comprises a magnetic tunnel junction (MJT) and includesferromagnetic layers separated by an insulating dielectric. Electronstunnel through the dielectric, known as a tunnel barrier, from a firstferromagnetic layer to a second ferromagnetic layer. The direction ofthe magnetization vectors in the ferromagnetic layers determines thetunneling resistance. A zero logic state is represented when themagnetization directions are parallel which corresponds to a lowtunneling resistance for the magnetic tunneling junction. Conversely, aone logic state is represented when the magnetization states areanti-parallel which corresponds to a high tunneling resistance.Typically, a magnetic vector in a first magnetic layer is fixed orpinned, while the magnetization direction of a second magnetic layer isfree to switch between the same and opposite (anti-parallel) directions.The memory is non-volatile because the ferromagnetic material holds themagnetization vectors when the memory is not powered. It should be notedthat the selection of the parallel state or the anti-parallel state as alogic one or zero state is arbitrary.

Typically, in MRAM and related magnetic sensor technology, the fixedlayer is a pinned synthetic anti-ferromagnet (SAF). SAF structures arewell known in literature and generally comprise two ferromagnetic layersof equal magnetic moment, separated by a spacer layer that providesanti-ferromagnetic coupling between them. Due to the anti-ferromagneticcoupling, the moments of the ferromagnetic layers point in oppositedirections in the absence of an applied field. The strength of the SAPis typically expressed in terms of the saturation field H_(sat), whichis the field needed to force the moments of the layers parallel to eachother. A unique feature of a SAF with a well-defined magnetic anisotropyis the flop behavior. As an external field increases, the momentssuddenly turn perpendicular to the field when it reaches a criticalvalue called the flop field H_(flop). Practically, the moments of theferromagnetic layers start to move in the vicinity of the flop field.Flop field is determined by the uniaxial anisotropy (H_(k)) of thelayers and the saturation field (H_(sat)) of the SAF. Uniaxialanisotropy is the field needed to saturate the magnetic moment of a filmalong its hard axis. To increase the field where the SAP starts to move,or in other words, the moments of the layers start to move, it is acommon practice in the industry to use a pinning layer. The pinninglayer fixes the moment of the FM layer adjacent to it in a particulardirection, which in turn sets the direction of the other layer in theSAF. Typical pinning layers used are Mn-based alloys, such as IrMn,PtMn, etc. A high temperature anneal is needed for the pinning layer topin the FM layer adjacent to it. The pinned SAF has certaindisadvantages and problems associated with it. Some of the reliabilityproblems are associated with Mn diffusion from the pinning layer. Also,the alloy typically used as the pinning material, PtMn, is very costlyand needs a very high temperature anneal for pinning, thereby increasingthe thermal budget. Addition of a layer always adds complexity to theMTJ stack. A fixed SAF with no pinning layer overcomes and embodies theabove mentioned problems and advantages.

The use of unpinned SAFs as a fixed layer is disclosed in U.S. Pat. No.5,583,725; however, the patent does not describe the specific alloys andparticular material stacks that are needed for the toggle MRAM describedin U.S. Pat. No. 6,545,906, that certain magnetic criteria need to bemet regardless of which specific alloy is used, or that the fixed layerneeds to be set in a particular direction for all the bits for properdevice operation.

The role of a fixed SAF is to remain rigid while the fields are used toswitch the free-layer, which can be a single free layer or a SAF(introduced by U.S. Pat. No. 6,545,906). The magnitude of the fieldwhere the free layer switches is defined as the switching field. If thefixed SAP magnetic moments rotate even slightly during switching, theswitching distributions of the free layer will be broadened and memoryoperation will be compromised. To be magnetically rigid enough forpractical use, the SAF structures should exhibit a high saturation fieldas well as a well-defined high flop field. A high and well-defined flopfield is an important requirement that is essential regardless of whichspecific alloy is used, as it ensures that the magnetic moments of thefixed SAF do not rotate while the free layer is switching.

Another issue associated with a pinning-free structure is finding a wayto set the direction of the fixed layer to be the same for all bits inall die on the wafer. This issue arises because, in the absence of apinning layer, the SAF is equally likely to be in either of the twostable magnetic states with the fixed layer moment either to the rightor to the left.

Accordingly, it is desirable to provide a pinning-free syntheticanti-ferromagnetic structure incorporating all the above requirementsfor use in conjunction with the toggle magnetoresistive random accessmemory array. Furthermore, other desirable features and characteristicsof the present invention will become apparent from the subsequentdetailed description of the invention and the appended claims, taken inconjunction with the accompanying drawings and this background of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a cross-sectional view of a magnetic tunnel junction bit inaccordance with an exemplary embodiment;

FIG. 2 is a simplified plan view of the magnetic tunnel junction bit ofFIG. 1 with bit and digit lines;

FIG. 3 is a partial cross-sectional view of an exemplary embodiment of amagnetic region within a magnetic tunnel junction bit; and

FIG. 4 is a partial cross-sectional view of another exemplary embodimentof a magnetic region within a magnetic tunnel junction bit.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

The exemplary embodiment described herein is a SAF structure exhibitinga well-defined high H_(flop) (flop field) using a combination of highH_(k) (uniaxial anisotropy), high H_(sat) (saturation field), and idealsoft magnetic properties exhibiting well-defined easy and hard axes. Theexemplary embodiment comprises an amorphous Cobalt-Iron-Boron-based(CoFeB) SAF grown on Tantalum (Ta), with a Cobalt-Iron (CoFe) insertionlayer at the bottom interface of the spacer to boost H_(sat). Thisstructure has been shown to have the combination of sufficiently highH_(sat) and well-defined high H_(k) with the desired soft magneticbehavior. This exemplary embodiment eliminates the need for a pinninglayer in MRAM devices.

CoFe-based alloys are generally known to exhibit high saturation fields;however, other alloy SAFs exhibiting desirable properties as describedherein can be used as well. A well-defined and high flop field is anessential requirement in addition to a high saturation field. The firststep is to grow soft/well-behaved (well-defined easy and hard axis, lowcoercivity and high H_(k)) single layer films of CoFe/CoFe-based alloys.Films with these above mentioned desirable properties are achieved whengrown on a Ta seed layer. Using the above mentioned seed, softCoFe-based films (CoFe, CoFeB with different Boron compositions) withcoercivities<5.0 Oe and H_(k)>30.0 M Oe are achievable.

SAFs grown with these alloys exhibit high H_(sat). CoFe provides thehighest H_(sat); however, at or near balance these SAFs did not exhibita well-defined H_(flop). The flop field shows up in amagnetization-versus-field (M-H) loop as a sharp slope change from aflat region at low field to a steep slope when the SAF movesapproximately 90 degrees to the applied field. With the CoFe SAFs, theflat-region disappears, indicating changes in the magnetizationdirection, something that broadens the free layer transition. AmorphousCoFeB SAFs, on the other hand, retain the flat, well-defined flop atbalance. When the boron composition in the CoFeB is decreased too much,the alloy becomes crystalline and the flat region is no longer welldefined, i.e., crystalline CoFeB behaves similar to CoFe and istherefore not suitable. These SAFs, which have no well-defined flop, arenot rigid and will move when a field is applied, though the saturationfields are high. Amorphous CoFeB (B>9%) with a thin CoFe at the bottominterface to boost H_(sat) exhibits desirable properties. H_(sat)preferably is greater than 1500 Oe. These SAFs exhibit similar MR as theones with pinned SAF. Having CoFe at one interface is desirable forincreasing H_(sat), but introducing CoFe on both the interfaces causedthe disappearance of the flop region similar to that observed in an allCoFe or crystalline CoFeB SAFs.

FIG. 1 is a simplified sectional view of an MRAM array 3 in accordancewith the exemplary embodiment. In this illustration, only a singlemagnetoresistive memory bit 10 is shown, but it will be understood thatMRAM array 3 includes a number of MRAM bits 10 and only one such bit isshown for simplicity.

MRAM bit 10 is sandwiched between a bit line 20 and a digit line 30. Bitline 20 and digit line 30 include conductive material such that acurrent can be passed there through. In this illustration, bit line 20is positioned on top of MRAM bit 10 and digit line 30 is positioned onthe bottom of MRAM bit 10 and is directed at a 90°angle to bit line 20as shown in FIG. 2.

MRAM bit 10 includes a free magnetic region 15, a tunneling barrier 16,and a fixed magnetic region 17, wherein tunneling barrier 16 issandwiched between free magnetic region 15 and fixed magnetic region 17.In this exemplary embodiment, free magnetic region 15 includes atri-layer structure 18, which has an anti-ferromagnetic coupling spacerlayer 65 sandwiched between two ferromagnetic layers 45 and 55.Anti-ferromagnetic coupling spacer layer 65 has a thickness 86 andferromagnetic layers 45 and 55 have thicknesses 41 and 51, respectively.Further, fixed magnetic region 17 has a tri-layer structure 19, whichhas an anti-ferromagnetic coupling spacer layer 66 sandwiched betweentwo ferromagnetic layers 46 and 58. The fixed magnetic region 17 isformed on a suitable seed layer 54 such as Tantalum. The magnetic bit 10contacts the bit line 30 and digit line 20 through bottom and topelectrodes (not shown). Anti-ferromagnetic coupling spacer layer 66 hasa thickness 87 and ferromagnetic layers 46 and 58 have thicknesses 42and 52, respectively.

Generally, anti-ferromagnetic coupling spacer layers 65 and 66 includeat least one of the elements Ruthenium, Rhodium, Chromium, Vanadium,Molybdenum, for example, or combinations thereof and alloys of thesesuch as Ruthenium-Tantalum. Further, ferromagnetic layers 45 and 55include at least one of elements Nickel, Iron, Cobalt, or combinationsthereof. In accordance with the exemplary embodiment, ferromagneticlayers 46 and 58 comprise Cobalt-Iron-Boron. Also, it will be understoodthat magnetic regions 15 and 17 can include material structures otherthan tri layer structures and the use of tri-layer structures in thisembodiment is for illustrative purposes only. For example, one suchmaterial structure could include a five-layer stack of a ferromagneticlayer/anti-ferromagnetic coupling spacer layer/ferromagneticlayer/anti-ferromagnetic coupling spacer layer! ferromagnetic layerstructure. Another exemplary material structure for layer 15 wouldinclude a single ferromagnetic layer.

Ferromagnetic layers 45 and 55 each have a magnetic moment vector 57 and53, respectively, that are usually held anti-parallel by coupling of theanti-ferromagnetic coupling spacer layer 65. Also, in some embodiments,the fixed magnetic region 17 has a resultant magnetic moment vector 50.The resultant magnetic moment vector 50 is oriented along an anisotropyeasy-axis in a direction that is at an angle, for example between 30°to60°, but preferably 45°, from bit line 20 and digit line 30 (see FIG.2). Further, magnetic region 15 is a free ferromagnetic region, meaningthat magnetic moment vectors 53 and 57 are free to rotate in thepresence of an appropriately applied magnetic field. Magnetic region 17is a fixed ferromagnetic region, meaning that the magnetic momentvectors 47 and 48 are not free to rotate in the presence of a moderateapplied magnetic field. Layer 46 is the reference layer, meaning thatthe direction of its magnetic moment vector 48 determines which freelayer magnetic states will result in high and low resistance.Ferromagnetic layer 58 having a magnetic moment 47, which could beeither a single layer or a double layer, comprises ferromagnetic layer56 and ferromagnetic interlayer 49, would be the pinned layer inconventional pinned SAF structures, but in this bit structure 10 the trilayer structure 19 is designed so that a pinning layer beneath layer 58is unnecessary.

The MRAM bit 10 does not have a pinning layer positioned contiguous toand for influencing the vector 50 within the fixed magnetic region 17.This is accomplished by fabricating the ferromagnetic layers 46 and 58so that they exhibit a well-defined and sufficiently-high H_(flop);using a combination of high H_(k) (uniaxial anisotropy), high H_(sat)(saturation field), and ideal soft magnetic properties exhibitingwell-defined easy and hard axes.

The fixed SAF with no pinning layer provides several advantages over theconventional pinned SAFs. Some of these advantages are: (a) SimplifiedMTJ stack, (b) reduced cost (PtMn, pinning material typically used isthe most expensive part of the MTJ stack), (c) an anneal step that isoptimized for the MTJ stack rather than the pinning material (which isusually a higher temperature and for a longer time) and (d) reduced Mndiffusion related problems.

The exemplary embodiment comprises an amorphous Cobalt-Iron-Boron-basedfixed SAF with different Boron compositions, grown on Tantalum, with aCoFe insertion layer 49 at the bottom interface of the spacer to boostH_(sat). The CoFe-based films preferably comprise coercivities of lessthan 5.0 Oe and H_(k) greater than 10 Oe and preferably greater than 15Oe. This structure has been shown to have the combination ofsufficiently high H_(sat) and well-defined high H_(k) with the desiredsoft magnetic behavior.

While anti-ferromagnetic coupling layers 65 and 66 are illustratedbetween the two ferromagnetic layers in each tri-layer structure 18 and19, it will be understood that the ferromagnetic layers 45, 55, 46, 58could he anti-ferromagnetically coupled through other means, such asmagnetostatic fields or other features. For example, when the aspectratio of a cell is reduced to five or less, the ferromagnetic layers areanti-parallel coupled from magnetostatic flux closure.

MRAM bit 10 has tri-layer structures 18 that have a length/width ratioin a range of 1 to 5 for a non-circular plan. It will be understood thatMRAM bit 10 can have various shapes, such as square, elliptical,rectangular, or diamond, but is illustrated in FIG. 2 as being circularfor simplicity.

Further, during fabrication of MRAM array 3, each succeeding layer (i.e.30, 54, 58, 66, 46, 16, 55, 65, etc.) is deposited or otherwise formedin sequence and each MRAM bit 10 may be defined by selective deposition,photolithography processing, etching, etc. in any of the techniquesknown in the semiconductor industry. During deposition of at least theferromagnetic layers 45 and 55, a magnetic field may be provided to seta preferred easy magnetic axis for this pair (induced anisotropy).Similarly, a strong magnetic held applied during the post-depositionanneal step may induce a preferred easy axis. In addition, the bits aretypically patterned to be longer in one direction, resulting in a shapeanisotropy that favors the long axis of the bit.

It is desirable to have the magnetic vector within the ferromagneticlayer 46 pointing in a particular direction. However, in case of abalanced SAF, this is difficult because the symmetric nature of thestructure means it is equally likely for the structure to end up ineither of the two stable states after the high-field anneal processstep. The measured resistance vs. field (R-H) loops for perfectlybalanced SAFs illustrate that, since some curves have increasingresistance as the external field H is turned up and others havedecreasing resistance with increasing H, some of the bottom SAFs set inone direction and some in the other.

This is controllable by introducing symmetry breakers in the referenceSAF. A slightly magnetically imbalanced structure makes the fixed SAFsfall into a preferred state when removed from the high-field used forthe high-field anneal step. An imbalance may be accomplished by makingone layer slightly thicker than the other, generating a net magneticmoment 50 for the trilayer structure 10. A small amount of imbalance issufficient to make the fixed layer in all the bits to go to the samedirection. Too much imbalance in the SAF is not preferred due to variousreasons. If the imbalance is too high, then we have a magnetic fieldacting on the bits from the imbalanced layer which influences devicecharacteristics, which is undesirable. Also, too much of an imbalancewill change flop behavior of the SAF. For best operation, the imbalanceshould be between 0.5% and 10%.

Another structure involving symmetry breaking without imbalancing themoment comprises the fixed magnetic region 17 (SAF) where the intrinsicH_(k) is different for the two ferromagnetic layers 46 and 58 (see FIG.3). For example, different alloys of NiFe or CoFe can be chosen to givea factor of 2 to 4 difference in H_(k), where vector 21 comprises afirst H_(k) and vector 22 comprises a second H_(k). The H_(k) differencegives an anisotropy energy difference for the two layers. Therefore,even after saturation from a large external field, the layer with thehigher H_(k) will stay closer to the applied field direction so as tominimize energy. Then the lower H_(k) layer will be the one to reverse.

Yet another symmetry breaking structure for the fixed magnetic region 17comprises a triple SAP shown in FIG. 4. This structure consists of amiddle ferromagnetic layer 33 of thickness t_(i), and upper and lowerferromagnetic layers 35 and 31 respectively, of thickness t_(o) and t₂respectively. Anti-ferromagnetic coupling spacer layers 32 and 34 arepositioned between ferromagnetic layers 31 and 33, and ferromagneticlayers 33 and 35, respectively. For this structure, t₁≈2t₀≈2t₂. In thisway, the moment is approximately balanced, but the dipole energy for themiddle ferromagnetic layer 33 is twice that of either outerferromagnetic layer 31 and 35. In addition, the shape anisotropy for themiddle ferromagnetic layer 33 is twice that of the outer ferromagneticlayers 31 and 35. The net result is that after saturation, the thickermiddle ferromagnetic layer 33 remains pointing in the field directionwhile the thinner ferromagnetic outer layers reverse.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention, it being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

1-20. (canceled)
 21. A method of fabricating a syntheticanti-ferromagnetic (SAF) structure in a magnetic tunnel junction (MTJ)stack, comprising: forming a first ferromagnetic layer; forming a secondferromagnetic layer; forming a coupling layer between the firstferromagnetic layer and the second ferromagnetic layer; and forming aferromagnetic insertion layer between the first ferromagnetic layer andthe coupling layer.
 22. The method of claim 21, wherein theferromagnetic insertion layer comprises cobalt and iron.
 23. The methodof claim 21, wherein each of the first ferromagnetic layer and the thirdferromagnetic layer comprises cobalt, iron, and boron.
 24. The method ofclaim 21, wherein the coupling layer is anti-ferromagnetic.
 25. Themethod of claim 21, wherein the first and second ferromagnetic layerseach include an alloy having cobalt, iron, and boron, and the secondferromagnetic layer includes an alloy having a different boronconcentration than the first ferromagnetic layer.
 26. The method ofclaim 21, wherein forming the ferromagnetic insertion layer includesforming the ferromagnetic insertion layer on the first ferromagneticlayer.
 27. The method of claim 21, wherein forming the firstferromagnetic layer includes forming the first ferromagnetic layer usinga cobalt-boron alloy, and forming the second ferromagnetic layerincludes forming the second ferromagnetic layer using a cobalt-boronalloy on the coupling layer.
 28. The method of claim 21, wherein formingthe first and second ferromagnetic layers each include using acobalt-boron alloy, and the second ferromagnetic layer includes an alloyhaving a different boron concentration than the first ferromagneticlayer.
 29. The method of claim 21, wherein forming the coupling layerincludes forming the coupling layer on the ferromagnetic insertionlayer.
 30. The method of claim 21, wherein the second ferromagneticlayer has a different thickness than the first ferromagnetic layer. 31.A method of fabricating a fixed magnetic region in a magnetic tunneljunction (MTJ) stack, comprising: forming a first ferromagnetic layer ona seed layer; forming an insertion layer on the first ferromagneticlayer, the insertion layer comprising cobalt and iron; forming acoupling layer on the insertion layer, the coupling layer providinganti-ferromagnetic coupling between the first ferromagnetic layer and asecond ferromagnetic layer; and forming the second ferromagnetic layeron the coupling layer, wherein the resultant fixed magnetic region is anunpinned synthetic anti-ferromagnetic (SAF) structure.
 32. The method ofclaim 31, wherein the first ferromagnetic layer comprises cobalt andiron.
 33. The method of claim 32, wherein the first ferromagnetic layerfurther comprises boron.
 34. The method of claim 31, wherein theinsertion layer comprises cobalt and iron.
 35. The method of claim 34,wherein each of the first ferromagnetic layer and the secondferromagnetic layer comprises at least two or more of: cobalt, iron, andboron.
 36. The method of claim 31, wherein the first ferromagnetic layerincludes an amorphous crystalline structure.
 37. A method of fabricatinga magnetic tunnel junction (MTJ) stack, comprising: forming a freemagnetic region; forming a fixed magnetic region consisting of anunpinned synthetic anti-ferromagnetic (SAF) structure, wherein formingthe fixed magnetic region includes: forming a first layer including atleast cobalt and iron, wherein the first layer is formed on a seedlayer; forming a second layer including at least cobalt and iron;forming a coupling layer between the first layer and the second layer;and forming an insertion layer between the first layer and the couplinglayer; and forming an intermediate region between the free magneticregion and the fixed magnetic region.
 38. The method of claim 37,wherein each of the first layer and the second layer further includesboron.
 39. The method of claim 38, wherein the second layer includes adifferent boron concentration than the first layer.
 40. The method ofclaim 37, wherein the coupling layer is anti-ferromagnetic.